Spec. Matrices 2020; 8:160–171 Research Article Open Access Joyentanuj Das, Sachindranath Jayaraman, and Sumit Mohanty* Distance Matrix of a Class of Completely Positive Graphs: Determinant and Inverse https://doi.org/10.1515/spma-2020-0109 Received February 6, 2020; accepted May 14, 2020 Abstract: A real symmetric matrix A is said to be completely positive if it can be written as BBt for some (not necessarily square) nonnegative matrix B. A simple graph G is called a completely positive graph if every matrix realization of G that is both nonnegative and positive semidenite is a completely positive matrix. Our aim in this manuscript is to compute the determinant and inverse (when it exists) of the distance matrix of a class of completely positive graphs. We compute a matrix R such that the inverse of the distance matrix of a class of completely positive graphs is expressed a linear combination of the Laplacian matrix, a rank one matrix of all ones and R. This expression is similar to the existing result for trees. We also bring out interesting spectral properties of some of the principal submatrices of R. Keywords: Completely positive graphs, Schur complement, Laplacian matrix, Distance matrix MSC: 05C12, 05C50 1 Introduction and Preliminaries Let G = (V, E) be a nite, connected, simple and undirected graph with V as the set of vertices and E ⊂ V × V as the set of edges. We write i ∼ j to indicate that the vertices i, j are adjacent in G. The degree of the vertex i is denoted by δi. A graph with n vertices is called complete, if each vertex of the graph is adjacent to every other vertex and is denoted by Kn. A graph G = (V, E) said to be bipartite if V can be partitioned into two subsets V1 and V2 such that E ⊂ V1 × V2. A bipartite graph G = (V, E) with the partition V1 and V2 is said to be a complete bipartite graph, if every vertex in V1 is adjacent to every vertex of V2. The distance d(i, j) from i and j in G is the length of the shortest path from i and j. The distance matrix of graph G is the n × n matrix, denoted by D(G) = [dij], where dij = d(i, j), if i ≠ j, and 0 if i = j. This denition requires G to be connected. The Laplacian matrix of G is the n × n matrix, denoted as L(G) = [lij], where lij = δi if i = j, −1 if i ≠ j, i ∼ j and 0 otherwise. Let T be a tree with n vertices. In [6], the authors proved that the determinant of the distance matrix D(T) of T is given by det D(T) = (−1)n−1(n − 1)2n−2. Note that, the determinant does not depend on the structure of the tree but the number of vertices. In [7], it was shown that the inverse of the distance matrix of a tree is −1 1 1 t t given by D(T) = − L(T) + ττ , where τ = (2 − δ , 2 − δ , ..., 2 − δn) . The above expression gives 2 2(n − 1) 1 2 a formula for the inverse of the distance matrix of a tree in terms of the Laplacian matrix. The determinant and the inverse of the distance matrix were also studied for bi-directed trees and weighted trees (for details, see [3, 10]). In [2], similar results were studied for q-analogue of the distance ma- Joyentanuj Das: School of Mathematics, IISER Thiruvananthapuram, Maruthamala P.O., Vithura, Thiruvananthapuram, Kerala- 695 551, India, E-mail: [email protected] Sachindranath Jayaraman: School of Mathematics, IISER Thiruvananthapuram, Maruthamala P.O., Vithura, Thiruvanantha- puram, Kerala- 695 551, India, E-mail: [email protected], [email protected] *Corresponding Author: Sumit Mohanty: School of Mathematics, IISER Thiruvananthapuram, Maruthamala P.O., Vithura, Thiruvananthapuram, Kerala- 695 551, India, E-mail: [email protected], [email protected] Open Access. © 2020 J. Das et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution alone 4.0 License. Distance Matrix of a Class of Completely Positive Graphs: Determinant and Inverse Ë 161 6 5 5 4 4 4 3 3 3 1 2 1 2 1 2 Figure 1: T4, T5, T6 trix, which is a generalization of the distance matrix for a tree. The inverse of the distance matrix has been explored for graphs such as block graphs, bi-block graphs and cactoid digraph (for details, see [4, 8, 9]). In this article, we study the determinant and inverse of the distance matrix for certain classes of completely positive graphs. 1.1 Completely Positive Graphs A matrix is said to be doubly nonnegative if it is both entrywise nonnegative and positive semidenite. A real symmetric matrix A is called completely positive (cp-matrix) if it can be decomposed as A = BBt, where B is a (not necessarily square) nonnegative matrix. Given an n × n symmetric matrix A, we can associate a graph G(A) with the matrix. The vertices of G(A) are 1, 2, ... , n and i ∼ j in G(A) if and only if i ≠ j and aij ≠ 0. Denition 1.1. A graph G is said to be a completely positive graph (cp-graph), if every doubly nonnegative, symmetric matrix A whose graph is G is completely positive. A vertex v of a connected graph G is a cut vertex of G if G − v is disconnected. A block of the graph G is a maximal connected subgraph of G that has no cut-vertex. There are many equivalent conditions to prove that a graph G is completely positive. We state a few of these. For n ≥ 3, let Tn be a graph consisting of (n − 2) triangles and a common base (see Figure 1). Some of the equivalent conditions are stated below. Theorem 1.2. [5, Corollary 2.6] The following properties of a graph G are equivalent: (i) G is a cp-graph. (ii) Each block of G is a cp-graph. (iii) Each block of G is either bipartite, or a K4, or a Tn. A graph said to be a bi-block graph if each of its blocks is a complete bipartite graph. Note that every tree is a bipartite graph. The determinant and inverse of the distance matrix have been studied for these graphs, and interestingly, the formula for the inverse of the distance matrix comes in terms of the Laplacian matrix (or the Laplacian like matrix; for details, see [6, 7, 9]). In view of Theorem 1.2, both bi-block graphs and trees are cp- graphs. In this article, our primary interest is to compute the determinant and inverse of the distance matrix of a class of cp-graphs such that each of its blocks is Tn, for a xed n and with a central cut vertex, which is (b) not a base vertex. We denote such graph as Tn , where b (≥ 2) represent number of blocks (see Figure 2). (b) (b) Let D(Tn ) denote the distance matrix of Tn . It turns out that, similar to the case of bi-block graphs (b) −1 (b) (b) and trees, D(Tn ) can be expressed in terms of the Laplacian matrix L(Tn ) and a new matrix R(Tn ). In (b) −1 particular, for n ≠ 6 and b ≥ 2, the expression for D(Tn ) is given by (b) −1 1 (b) 1 1 (b) D(Tn ) = − L(Tn ) + J + R(Tn ), (1.1) 2 2b 2(n − 6)b 162 Ë Joyentanuj Das, Sachindranath Jayaraman, and Sumit Mohanty 6 7 8 12 5 6 9 7 10 5 6 16 15 14 13 8 8 4 9 5 4 4 11 9 3 3 7 3 1 2 1 2 1 2 Figure 2: T(4) T(2) T(3) 3 , 5 , 6 (b) where J is a matrix of all one’s. The explicit denition of R(Tn ) is given in Section 3. Unlike the case of trees (b) −1 and bi-block graphs, we only give an expression for D(Tn ) similar to the trees. At this point, we are not able to retrieve information about the cp-graph from the matrix R. This article is organized as follows. In Section 1.2, we recall necessary results from matrix theory and also x a few notations which will appear throughout this article. In Section 2, we compute the determinant and inverse of the distance matrix of single blocks of Tn. Since results on the inverse of the distance matrix of a complete bipartite graph have appeared in the work of Hou et al [9], we skip these, although it is possible to present proofs that is completely dierent from theirs. In Section 3, we compute the determinant of the (b) distance matrix of Tn and nd its inverse, whenever it exists. The manuscript ends with a section on spectral (b) properties of some principal submatrices of the matrix related to R(Tn ), appearing in Eqn (1.1). 1.2 Notations and Some Preliminary Results We begin this section by introducing a few notations which will be used throughout this article. Let In and 1 n , denote the identity matrix and the column vector of all ones of order n respectively. Further, Jm×n denotes the m × n matrix of all ones and if m = n, we use the notation Jm. We write 0m×n to represent zero matrix of order m × n. Unless there is a scope for confusion, we omit the order of the matrices.